The present invention relates to a method of manufacturing a silica glass member and a silica glass member obtained by the method and, more particularly, to a method of manufacturing a silica glass member suitably used for an imaging optical system such as an illumination optical system or projection optical system in a projection exposure apparatus for transferring a predetermined mask pattern onto a substrate, and a silica glass member obtained by the method.
As a projection exposure apparatus, an apparatus having a structure as shown in FIGS. 18A and 18B is conventionally used.
More specifically, in the projection exposure apparatus shown in FIG. 18A, a light beam from a light source 501 such as a mercury arc lamp is condensed by an elliptical mirror 502 and then converted into a parallel light beam by a collimator lens 503. This parallel light beam passes through a fly-eye lens 504 comprised of a set of optical elements 504 a each having a rectangular section as shown in FIG. 18, so a plurality of light source images are formed on the exit side of the fly-eye lens. An aperture stop 505 having a circular aperture portion is placed at the light source image position. Light beams from the plurality of light source images are condensed by a condenser lens 506 so a reticle R as a target illumination object is uniformly illuminated in a superposed manner.
A pattern on the reticle R thus uniformly illuminated with the illumination optical system is projected/exposed to a wafer W with a resist coated thereon, by a projection optical system 507 constituted by a plurality of lenses. This wafer W is placed on a wafer stage WS that two-dimensionally moves. In the projection exposure apparatus shown in FIG. 18A, exposure of so-called step-and-repeat scheme is performed so that when exposure in one shot region on the wafer is ended, the wafer stage is sequentially two-dimensionally moved for exposure to the next shot region.
In recent years, a scanning exposure scheme has been proposed in which the pattern on the reticle R can be transferred onto the wafer W at high throughput by irradiating the reticle R with a rectangular or arc light beam and scanning the reticle R and wafer W, which are conjugate with respect to the projection optical system 507, in predetermined directions.
In either projection exposure apparatus, an optical member used in its optical system is required to have a high transmittance for exposure light used. This is because the optical system of the projection exposure apparatus is formed by combining a number of optical members, and if optical losses of the number of optical members used are integrated, the influence of total decrease in transmittance becomes large althrough the optical loss per lens is small. When an optical member having a low transmittance is used, it absorbs exposure light and increases the temperature of the optical member, resulting in uneven refractive index. Additionally, the polished surface deforms due to local thermal expansion of the optical member. This degrades the optical performance.
In the projection optical system, the optical member is required to have a highly uniform refractive index in order to obtain a finer and clearer projected/exposed pattern. The reason for this is that a propagation delay of light occurs due to an unenen refractive index, greatly affecting the imaging performance of the projection optical system.
As a material of an optical member used in the optical system of a projection exposure apparatus using UV light (wavelength: 400 nm or less), silica glass or calcium fluoride crystal, which has a high transmittance for UV light and is excellent in uniformity, is generally used.
Furthermore, a technique has been recently proposed in which the wavelength of the light source is shortened to transfer a finer mask pattern image onto a wafer surface, i.e., improve the resolution. For example, the wavelength is shortened from conventional g line (wavelength: 436 nm) or i line (wavelength: 365 nm) to KrF (wavelength: 248 nm) or ArF (wavelength: 193 nm) excimer laser.
In projection exposure using such a short-wavelength excimer laser, since it aims at obtaining a finer mask pattern, a material having more excellent characteristics for the transmittance or uniformity of refractive index is used.
However, even a material having high and uniform transmittance and refractive index does not always present a desired resolution when a plurality of materials are combined to form an optical system.
The present invention has been made in consideration of the above problems of the prior art, and has as its object to provide a method of manufacturing a silica glass member, which makes it possible to efficiently and properly obtain a silica glass member necessary for obtaining a sufficiently high resolution in the imaging optical system of a projection exposure apparatus, and a silica glass member obtained by the manufacturing method.
As a result of extensive studies aiming at achieving the above object, the present inventors have found that the imaging performance of a projection optical system and the resolution of a projection exposure apparatus are affected by the birefringence of an optical member, and when the magnitude of the birefringence, i.e., the birefringence value (absolute value) of an optical member is 2 nm/cm or less, and the distribution of birefringence values in the optical member has a central symmetry, imaging performance close to the designed performance of the projection optical system and a resolution close to the designed performance of the projection exposure apparatus are obtained, and have disclosed it in Japanese Patent Laid-Open No. 8-107060.
However, when the required resolution of a projection exposure apparatus further rises, and light having a shorter wavelength is used as exposure light, or an optical member having a large diameter and thickness is used, no satisfactory imaging performance of the projection optical system and no satisfactory resolution of the projection exposure apparatus can be obtained even by employing the conventional design concept.
As a result of more extensive studies, the present inventors have found, as a reason why a projection optical system and projection exposure apparatus having desired optical performance cannot be obtained even by using an optical member having a satisfactory transmittance or satisfactorily uniform refractive index, that since the optical members have birefringence value distributions, respectively, and the birefringence value distributions are integrated in the entire optical system when a plurality of optical members are combined as a projection optical system, the light wavefront of the entire optical system is disturbed, adversely affecting the imaging performance of the projection optical system or resolution of the projection exposure apparatus.
More specifically, the conventional evaluation of the birefringence value of an optical member is done only on the basis of its magnitude (absolute value), and there is no concept of the birefringence value distribution of an optical member. For example, to measure the birefringence value of a silica glass member, birefringence values are measured at several points near 95% of the diameter of the member, and the maximum value is used as the birefringence value of the member, as recognized by those who skilled in the art. However, the present inventors measured the distribution of birefringence values of a silica glass member in detail and found that the birefringence values actually have an uneven distribution.
Hence, even for a silica glass member having a highly uniform refractive index, the influence of birefringence in the member cannot be sufficiently evaluated only by managing the maximum value of the birefringence values in the member. Especially, it is very hard to obtain an optical system having desired performance by combining a plurality of members.
As described above, since the evaluation of birefringence in the entire optical system constituted by a plurality of optical members cannot be simply represented only by the magnitudes (absolute values) of the birefringence values of the respective optical members, the present inventors examined in detail the influence that is given to the optical system by the uneven birefringence value distributions in the optical members. The present inventors consequently found by examining the uneven birefringence value distribution in an optical member in consideration of the direction of phase advance axis that in synthesis of silica glass by a direct method, and subsequent annealing or high-temperature heat treatment, it is difficult to control the direction of phase advance axis in the birefringence value distribution of a silica glass member by the conventional method, and also since a plurality of silica glass members obtained by this method have birefringence value distributions with the same phase advance axis direction, the birefringence values are integrated to adversely affect an optical system constituted using these members. The present inventors also found that by controlling a silica glass ingot obtained in the silica glass member manufacturing process to have a specific temperature distribution, a silica glass member having a birefringence value distribution with a phase advance axis direction different for that of the silica glass member obtained by the above congenital manufacturing method can be obtained, and completed the present invention.
More specifically, the first silica glass member manufacturing method of the present invention comprises:
a first step of making a silicon compound react in oxyhydrogen flame using a burner having a multi-tubular structure to obtain fine silica glass particles;
a second step of depositing the fine silica glass particles on a support rotating and placed to oppose the burner to obtain a silica glass ingot with a temperature distribution in at least one plane perpendicular to a rotational axis of the silica glass ingot, the temperature distribution being symmetrical with respect to the rotational axis and having a maximal value between a center and a peripheral portion of the plane; and
a third step of obtaining a distribution of signed birefringence values on the basis of birefringence values and directions of phase advance axes measured at a plurality of points in the plane perpendicular to the rotational axis of the silica glass ingot and cutting, from the silica glass ingot, a silica glass member whose signed birefringence values monotonously increase from the center to the peripheral portion of the plane.
The second silica glass member manufacturing method of the present invention comprises:
a fourth step of heating a silica glass ingot to a predetermined temperature;
a fifth step of cooling the silica glass ingot with a temperature distribution in at least one plane perpendicular to a rotational axis of the silica glass ingot, the temperature distribution being symmetrical with respect to the rotational axis and having a maximal value between a center and a peripheral portion of the plane; and
a sixth step of obtaining a distribution of signed birefringence values on the basis of birefringence values and directions of phase advance axes measured at a plurality of points in the plane perpendicular to the rotational axis of the silica glass ingot and cutting, from the silica glass ingot, a silica glass member whose signed birefringence values monotonously increase from the center to the peripheral portion of the plane.
The first silica glass member of the present invention is a silica glass member having a distribution of signed birefringence values, in which the signed birefringence values monotonously increase from a center to a peripheral portion of the plane, the silica glass member being obtained by a manufacturing method comprising:
a first step of making a silicon compound react in oxyhydrogen flame using a burner having a multi-tubular structure to obtain fine silica glass particles;
a second step of depositing the fine silica glass particles on a support rotating and placed to oppose the burner to obtain a silica glass ingot with a temperature distribution in at least one plane perpendicular to a rotational axis of the silica glass ingot, the temperature distribution being symmetrical with respect to the rotational axis and having a maximal value between the center and the peripheral portion of the plane; and
a third step of obtaining a distribution of signed birefringence values on the basis of birefringence values and directions of phase advance axes measured at a plurality of points in the plane perpendicular to the rotational axis of the silica glass ingot and cutting, from the silica glass ingot, a silica glass member whose signed birefringence values monotonously increase from the center to the peripheral portion of the plane.
The second silica glass member of the present invention is a silica glass member having a distribution of signed birefringence values, which monotonously increase from a center to a peripheral portion of the plane, the silica glass member being obtained by a manufacturing method comprising:
a fourth step of heating a silica glass ingot to a predetermined temperature;
a fifth step of cooling the silica glass ingot with a temperature distribution in at least one plane perpendicular to a rotational axis of the silica glass ingot, the temperature distribution being rotationally symmetrical with respect to the center of the plane and having a maximal value between the center and the peripheral portion of the plane; and
a sixth step of obtaining a distribution of signed birefringence values on the basis of birefringence values and directions of phase advance axes measured at a plurality of points in the plane perpendicular to the rotational axis of the silica glass ingot and cutting, from the silica glass ingot, a silica glass member whose signed birefringence values monotonously increase from the center to the peripheral portion of the plane.
According to the present invention, control is performed in the silica glass member manufacturing process such that the silica glass ingot has the specific temperature distribution, the distribution of signed birefringence values is obtained on the basis of birefringence values and their directions of phase advance axes measured at a plurality of points in a predetermined plane of the resultant silica glass ingot, and a silica glass member whose distribution of signed birefringence values monotonously increase from the plane center to the peripheral portion is cut, thereby efficiently and properly obtaining a silica glass member having a distribution of birefringence values, in which the birefringence values monotonously increase from the plane center to the peripheral portion. When the silica glass member of the present invention obtained in the above way and a conventional silica glass member having a distribution of signed birefringence values, in which the signed birefringence values monotonously decrease from the plane center to the peripheral portion are used, the birefringence values are sufficiently uniformed while realizing high transmittance and highly uniform refractive index in the entire optical system. Hence, a sufficiently high resolution can be obtained in the imaging optical system of a projection exposure apparatus.
The concept of a signed birefringence value according to the present invention will be described here.
A signed birefringence value means a birefringence value to which a sign is given in consideration of the direction of phase advance axis defined in an index ellipsoid in obtaining the birefringence value of an optical member.
More specifically, in a plane which is centered on the intersection to the optical axis of an optical member and is perpendicular to the optical axis, a region circularly irradiated with a light beam is defined as an almost circular effective cross-section. When the direction of phase advance axis in a small region at a birefringence measuring point on this effective cross-section is parallel to the radiation direction from the center as the intersection to the optical axis of the optical member, the plus sign is given to the measured birefringence value. When the direction of phase advance axis is perpendicular to the radiation direction, the minus sign is given to the measured birefringence value.
The manner a sign is given to a birefringence value is also applicable when the plane which is centered on the intersection to the optical axis of the optical member and is perpendicular to the optical axis is irradiated with a plurality of light beams. In this case as well, when the radiation direction from the center as the intersection to the optical axis of the optical member is parallel to the direction of phase advance axis in a small region at a birefringence measuring point on an effective cross-section irradiated with each of the plurality of light beams, the plus sign is given to the measured birefringence value. When the direction of phase advance axis is perpendicular to the radiation direction, the minus sign is given to the measured birefringence value.
The manner a sign is given to a birefringence value is also applicable when the plane which is centered on the intersection to the optical axis of the optical member and is perpendicular to the optical axis is irradiated with a light beam having a sectional shape other than a circular sectional shape, e.g., a light beam having a ring-shaped cross-section or elliptical cross-section. In this case as well, when the radiation direction from the center as the intersection to the optical axis of the optical member is parallel to the direction of phase advance axis in a small region at a birefringence measuring point on an effective cross-section irradiated with each of the plurality of light beams, the plus sign is given to the measured birefringence value. When the direction of phase advance axis is perpendicular to the radiation direction, the minus sign is given to the measured birefringence value.
In the following description, a case wherein the plus sign is given to a birefringence value measured when the direction of phase advance axis in a small region at a birefringence measuring point on the effective cross-section irradiated with a light beam is parallel to the radiation direction from the center as a the intersection to the optical axis of the optical member, and the minus sign is given to a birefringence value measured when the direction of phase advance axis is perpendicular to the radiation direction will be described.
The signed birefringence value will be described below in more detail with reference to FIGS. 1A, 1B, 2A, 2B, 3A, and 3B.
FIG. 1A is a schematic diagram to show the directions of phase advance axes at birefringence measuring points P11, P12, P13, and P14 separated from a center O on the effective cross-section of an optical member L1 by distances r1, r2, r3, and r4, respectively. Referring to FIG. 1A, the birefringence measuring points P11 to P14 are set on a line Q1 radially extending through the center O1, for the descriptive convenience. The size of a small region represented by a circle at each measuring point corresponds to the optical path difference at the measuring point. Line segments W11, W12, W13, and W14 in these small regions represent the directions of phase advance axes. Since the directions of phase advance axes at all the measuring points P11 to P14 are parallel to the direction of line Q1, i.e., the radial direction, all the birefringence values at the measuring points P11 to P14 are expressed with the plus sign. The radial distribution of signed birefringence values A11, A12, A13, and A14 at the measuring points P11 to P14 shown in FIG. 1A, which are obtained in the above way, has a profile shown in, e.g., FIG. 1B.
FIG. 2A is a schematic diagram to show the directions of phase advance axes at birefringence measuring points P21, P22, P23, and P24 separated from a center O2 on the effective cross-section of an optical member L2 by the distances r1, r2, r3, and r4, respectively, like FIG. 1A. In this case, since the directions of phase advance axes W21, W22, W23, and W24 at all the measuring points P21 to P24 are perpendicular to the direction of line Q2, i.e., the radial direction, all signed birefringence values A21, A22, A23, and A24 at the measuring points P21 to P24 are expressed with the minus sign. The radial distribution of the signed birefringence values A21 to A24 at the measuring points P21 to P24 shown in FIG. 2A, which are obtained in the above way, has a profile shown in, e.g., FIG. 2B.
FIG. 3B is a schematic diagram to show the directions of phase advance axes at birefringence measuring points P31, P32, P33, and P34 separated from the center O on the effective cross-section of an optical member L2 by the distances r1, r2, r3, r4, and r5 respectively, like FIG. 1A. In this case, as for the directions of phase advance axes W31, W32, W33, W34, and W35 at the measuring points P11 to P14, since the directions of phase advance axes are parallel to the direction of line Q3, i.e., the radial direction at the measuring points P31 to P33 and perpendicular to the radial direction at the measuring points P33 and P34, the radial distribution of signed birefringence values A31 to A35 obtained at the measuring points P31 to P35 has a profile as shown in FIG. 3B.